Electrical transformers are generally constructed of two coils of conductor (generally known as the primary and secondary windings or coils) wound about a core. The core is often constructed of a series of stacked thin steel plates which are wrapped in an insulating material. The individual windings about the core are also insulated so as to prevent an electrical short between adjacent windings or layers. (The conventional aspects of transformer design are set forth in the McGraw-Hill Encyclopedia of Engineering (1983) at pages 1115-1120, the disclosure of which is hereby incorporated by reference.)
The presence of excessive voids and gaps between and among the layers and windings of conductor constitute regions of vulnerability at which the electromagnetic forces typically generated within a transformer can cause damage. Spatial limitations also often impose constraints upon transformer design. Therefore, it is often important that the transformer be as compact and space efficient as possible. This has led designers to utilize cores of rectangular cross-section about which are wound conductor having a square or rectangular cross-section
The use of rectangular conductors about a rectangular core is space efficient. However, it presents problems of its own. When a rectangular conductor is bent 90 degrees through a sharp turn about one of the corners of the core, it undergoes deformation in the region of the bend. When a rectangular conductor is bent about a small radius (which is the case for the conductor wound immediately adjacent the core), the longitudinally oriented portions of the conductor that lie nearer the center of curvature become shorter. Concomitantly, the longitudinally oriented portions of the conductor that are farther removed from the center of curvature become elongated. The area where the two portions meet undergoes neither elongation nor foreshortening along the longitudinal axis of the conductor (in cross-section, this is the neutral axis). Because the overall volume of the conductor remains generally constant, changes in the dimension of the conductor in the longitudinal direction have countervailing effects in the cross-sectional shape of the conductor. At the inner radius, the conductor is placed in compression and a compressive strain in the axial direction of the conductor occurs. The axial strain is known to occur along with a lateral strain. Within the elastic limit of the material, these strains are proportional to each other (the well-known Poisson's ratio). As the material is bent further, the stresses exceed the elastic limit and the material acquires a permanent deformation. Where the longitudinal fibers have elongated (the outer portion), the cross-sections constrict and become smaller; where the axial fibers have shortened (the inner portion), the cross-section expands and becomes larger (the "mushroom" effect). The overall effect in this plastic regime is that the conductor shape that was originally rectangular in cross-section appears to "mushroom out" at the side nearer the core and the conductor acquires the shape of a trapezoid at the bend. The result of this process is illustrated in FIG. 1--the rectangular cross-section has become a trapezoid. Unless accounted for, this trapezoidal distortion may cause interference with the conductor immediately adjacent it in the coil. This mushrooming effect can result in the dielectric failure of the insulation about the conductor as the wider base of the trapezoid pinches through the insulation wrapped about it or about an adjacent turn of the conductor.
In the prior art, this effect has been accommodated for by allowing increased spacing between conductor windings, thereby reducing the compactness of the coil. There remains a need for a conductor winding which is both space efficient and which more reliably maintains the dielectric integrity of the coil.